A conductor is a material through which electrical current can flow easily. Conductors are usually composed of aluminum, copper, or copper-covered steel. Aluminum is light and low cost, but also has a lower conductivity than copper. It is commonly used for large electrical cables and underground residential distribution (URD) cables. Copper is the most common conductor material because it has a high conductivity and is strong, flexible, and easy to shape. To prevent or reduce oxidation, copper conductors may be coated with tin, an inexpensive and easy to solder material. Tin-coated copper wires have a shiny silver appearance. Copper-covered steel is a similar combination of metals often used in coaxial cables.